Cellular wireless is an increasingly popular means of personal communication in the modern world. People are using cellular wireless systems for the exchange of voice and data over such devices as cellular telephones, personal digital assistants (PDAs), cellular modems, and other mobile stations. In principle, a user can seek information over the Internet or call anyone over the Public Switched Telephone Network (PSTN) from any place inside the coverage area of the cellular wireless system.
In a typical cellular wireless system, an area is divided geographically into a number of cells provided by a radio access network (RAN). The RAN typically comprises one or more base transceiver stations (BTSs), each of which has one or more antennas that radiate to define a radio frequency (RF) radiation pattern. The BTS(s) of the RAN may then be coupled with a base station controller (BSC) or radio network controller (RNC), which may in turn be coupled with a telecommunications switch or gateway, such as a mobile switching center (MSC) or packet data serving node (PDSN) for instance. The switch or gateway may then provide connectivity with a transport network, such as the PSTN or the Internet. When a mobile station (such as a cellular telephone, a wirelessly equipped PDA or personal computer, or another suitably equipped device) is positioned in a cell, the mobile station communicates via an RF air interface with the BTS of the cell. Consequently, a communication can be established between the mobile station and another entity, via the RF air interface and the RAN.
With the explosive growth in demand for wireless communications, the level of call traffic in most cells has increased drastically over recent years. To help manage the call traffic, most cells in a wireless network are usually further divided geographically into a number of sectors, each defined respectively by radiation patterns from directional antenna components of the respective BTS, or by respective BTS antennae. Herein, cells and sectors are sometimes referred to as “wireless coverage areas.”
In a Code Division Multiple Access (CDMA) wireless system and perhaps in other types of systems, each cell employs one or more carrier frequencies, and each sector is distinguished from adjacent sectors by a pseudo-random number offset (PN offset). Further, each sector may concurrently communicate on multiple different channels, distinguished by “Walsh codes”. When a mobile station operates in a given sector, communications between the mobile station and the BTS of the sector are carried on a given frequency and are encoded by the sector's PN offset and, perhaps, a given Walsh code.
According to well known industry standards, a mobile station can communicate with a number of “active” sectors at a time. Depending on the system, the number of active sectors may be up to three or six, for instance. The mobile station receives largely the same signal from each of the active sectors and, on a frame-by-frame basis, may select the best signal to use. A typical mobile station maintains in its data storage a list of the sectors in its active set (the “active set members”). In addition, the mobile station maintains a list of “neighbor” sectors, which are those sectors that are not in the active set but that are in close vicinity to the mobile station (e.g., those sectors neighboring the mobile station's active set members). These neighbor sectors are collectively referred to as the mobile station's “neighbor set.”
In existing systems, to facilitate a determination of which sectors should be in the mobile station's active set, all base stations emit a pilot channel signal on each sector, typically at a power level higher than other downlink signals. A mobile station then regularly measures the strength (e.g., Ec/Io or signal-to-noise ratio) of each pilot signal that it receives and notifies the RAN when the strength of a pilot signal rises above or falls below respective designated thresholds. The RAN, in turn, provides the mobile station with an updated list of active set members.
In one arrangement, for instance, the RAN may transmit to the mobile station (e.g., over a downlink control channel or traffic channel) a Handoff Direction Message (HDM), containing parameters such as (i) the PN offsets of the mobile station's active set members and (ii) the following handoff parameters that relate to pilot signal strength:                T_ADD: Threshold pilot signal strength for addition to the active set (e.g., −14 dB)        T_COMP: Difference in signal strength from an active set member pilot (e.g., 2 dB)        T_DROP: Threshold pilot signal strength for removal from the active set (e.g., −16 dB)        T_TDROP: Time for which an active set member pilot falls below T_DROP to justify removal from the active set (e.g., 2 seconds)        
Additionally, the RAN may provide the mobile station with a Neighbor List Update Message (NLUM), containing a neighbor list that identifies the neighbor sectors of the mobile station's current active set members. In a CDMA system, the neighbor list may identify neighbor sectors at least in part by PN offset. The mobile station may then monitor all of the pilot signals that it receives and may determine if the pilot signal strength of any neighbor sector exceeds T_ADD by T_COMP. If so, the mobile station may send a Pilot Strength Measurement Message (PSMM) to the base station, indicating the estimated Echo for the neighbor sector, with the neighbor sector identified by PN offset. Depending on current capacity and other issues, the RAN may then agree to allow the mobile station to hand off to the neighbor sector. Accordingly, the RAN may reserve a channel resource (e.g., a Walsh code) in the neighbor sector and may send to the mobile station an HDM (i) providing a new active set that includes the sector having a pilot signal strength that exceeds T_ADD by T_COMP, and (ii) directing the mobile station to use the reserved channel resource in the added active set member. Further, the RAN may send to the mobile station a new NLUM containing a new neighbor list corresponding to the mobile station's revised active set.
After receipt of the HDM that provides the new active set, the mobile station may send a Handoff Completion Message (HCM) to the RAN, acknowledging the instruction, and providing a list of its active set members (identified by respective PN offsets), thereby completing the handoff.
Similarly, if the mobile station detects that the pilot signal strength of an active set member drops below T_DROP, the mobile station may start a handoff drop timer. If T_TDROP passes, the mobile station may then send a PSMM to the RAN, indicating the Echo and drop timer, and similarly identifying the active set member by PN offset. The RAN may then respond by sending to the mobile station an HDM providing a new active set that does not include the sector having signal strength below T_DROP. Further, the base station may likewise send to the mobile station a new NLUM containing a new neighbor list.
In typical practice, the neighbor list that the RAN provides to the mobile station will define a priority scanning order of the neighbor sectors listed in the neighbor list. The priority scanning order is usually defined in advance (e.g., by network engineers) based on the relative likelihood that the mobile station will hand off to the listed neighbor sectors. In operation, a mobile station will then cyclically scan for (i.e., monitor) pilot signals from the various sectors in its active and neighbor sets. In one implementation, for example, the mobile station may (i) scan all of its active sectors and then scan a first (highest priority) sector from its neighbor set, (ii) scan all of its active sectors again and then scan a next (next priority) sector from its neighbor set, and so forth until the mobile station has scanned all of its neighbor set sectors.